Magnetic resonance imaging (MRI) involves the transmission of radio frequency (RF) energy. RF energy may be transmitted by a coil. Resulting magnetic resonance (MR) signals may also be received by a coil. An electromagnetic coil, which may also be referred to as a “coil”, is formed when a conductor is wound around a core or form to create an inductor or electromagnet. The conductor may be, for example, a copper wire or trace. When electricity passes through the coil, it generates a magnetic field. One loop of a conductor may be referred to as a turn or a winding. A coil may include one or more turns. In early MRI, RF energy may have been transmitted from a single coil and resulting MR signals received by a single coil. Later, multiple receivers may have been used in parallel acquisition techniques. Using multiple receivers facilitates speeding up signal reception, which in turn may reduce scan time. Similarly, multiple transmitters may be used in parallel transmission techniques. Using multiple transmitters may facilitate speeding up a transmission process, which in turn may facilitate volumetric excitation, selective isolation, and other very high speed features.
Conventional systems may have been limited by their use of relatively low power (e.g., <50 W), low efficiency class A or class AB amplifiers. While some systems may have included on-coil series and/or shunt-fed class-D amplifiers, even these conventional systems have suffered from several limitations including inadequate detuning and low efficiency.
U.S. Pat. No. 7,671,595 ('595) issued to Griswold et al. on Mar. 2, 2010, which is entitled “On-coil Switched Mode Amplifier for Parallel Transmission in MRI” describes an on-coil current-mode class D (“CMCD”) amplifier that may be used to produce MRI transmission-coil excitations at desired RF frequencies. The on-coil CMCD amplifier is capable of performing within or proximate to the bore of the MRI magnet or within less than one wavelength of the amplifier from the transmit coil. Providing an on-coil amplifier allows digital control signals to be sent to the coil assembly, improving synchronization between the transmission-coils while reducing interference, cross talk, physical space requirements associated with cables, and heating normally associated with parallel transmission MRI systems.